This invention relates to an exhaust gas cleaning element especially designed for cleaning the exhaust gas emitted from the internal combustion engines of automobiles, etc., and a method of producing such an element.
There are known in the art the following methods for producing this type of exhaust gas cleaning element: (1) a method in which a flat sheet mainly composed of a ceramic material and a corrugated sheet obtained by corrugating said flat sheet are wrapped alternately around a center shaft to form an integral structure, and this structure is fired and solidified to obtain a desired exhaust gas cleaning element, and (2) a method in which a plastic batch material mainly composed of ceramic is fed under pressure into a molding die and extrusion molded to form a molding having a honeycomb structure, and this molding is fired and solidified as in the case of method (1) to obtain an exhaust gas cleaning element. The elements obtained according to these methods, however, involve the following problem: when such an element is set in the exhaust gas passage for effecting cleaning of the exhaust gas, although the exhaust gas flow resistance is low, the interaction between the exhaust gas and the wall surfaces of the element is also lowered because the direction of the exhaust gas flow is the same as the direction of the cells formed in the element, so that the material mobility between the flowing exhaust gas and the element wall surfaces is restricted to give rise to a so-called blow-by phenomenon of the exhaust gas, resulting in a poor exhaust gas cleaning efficiency and a low capturing rate of the fine particles (such as carbon particles) contained in the exhaust gas.
A method has been also proposed in which a specific configuration is made on the mold used for the extrusion molding of said honeycomb structure to form the projections or ruggedness on the inner wall surfaces of the produced honeycomb structure. However, this method, too, is unable to improve the interaction between exhaust gas and wall surfaces of the structure because the direction that the projections rise from the surface is inevitably the same as the exhaust gas flow direction owing to the use of extrusion molding for forming the honeycomb structure. This method also had the problem of low capturing rate of the fine particles existing in the exhaust gas and hence an unsatisfactory exhaust gas cleaning efficiency.
An exhaust gas cleaning element composed of a ceramic porous material having a plurality of voids opening to the outside for allowing passage of exhaust gas therethrough has been also proposed. This element using such ceramic porous material, however, is still unsatisfactory in capturing the fine particles such as carbon particles contained in exhaust gas.
An object of the present invention is to provide an improved exhaust gas cleaning element in which a plurality of projections are provided on the inner wall surfaces in said ceramic honeycomb structure or on the skeleton of a ceramic porous mass so as to disturb the exhaust gas flow by said projections to activate the interaction between the exhaust gas and the inner wall surfaces in said structure or skeleton of said ceramic mass and thereby enhance the exhaust gas cleaning action of the element.
Another object of the present invention is to provide an exhaust gas cleaning element having an improved exhaust gas cleaning performance, said element being characterized in that a plurality of through-holes are formed in the inner walls of a plurality of cells in a ceramic honeycomb structure.
A further object of this invention is to provide a method of producing such an exhaust gas cleaning element.